1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to chemical additives for improved end point signals for slurries for the chemical mechanical polishing (CMP) of thin films used in semiconductor integrated circuit manufacturing.
2. Background
Today, integrated circuits can be made up of literally millions of active devices formed in or on a silicon substrate. The active devices which are initially isolated from one another are later connected together to form functional circuits and components. The devices are typically interconnected together through the use of multilevel interconnections.
A cross-sectional illustration of a typical multilevel interconnection structure 100 is shown in FIG. 1. Interconnection structures typically have a first layer of metallization, an interconnection layer 102 (typically a copper alloy or an aluminum alloy with up to 3% copper), a second level of metallization 104, and sometimes a third, fourth or even higher level of metallization. Interlevel dielectrics 106 (ILDs), such as doped and undoped silicon dioxide (SiO2), are used to electrically isolate the different levels of metallization in silicon substrate or well 108.
The electrical connections between different interconnection levels are made through the use of metallized vias, such as metallized via 110 formed in ILD 106. In a similar manner, metal contacts such as metal contact 112, are used to form electrical connections between interconnection levels and devices formed in or on substrate or well 108. The metal vias 110 and contacts 112, hereinafter being collectively referred to as “vias” or “plugs”, are generally filled with tungsten 114 and generally employ an adhesion layer 116 such as titanium nitride (TiN). Adhesion layer 116 acts as an adhesion layer for the tungsten metal layer 114 which is known to adhere poorly to SiO2. At the contact level, the adhesion layer also acts as a diffusion barrier to prevent a reaction between tungsten and silicon of the substrate or well 108.
In one process for filling vias which has presently gained wide interest, metallized vias or contacts are formed by a blanket tungsten deposition and a chemical mechanical polish (CMP) process. In a typical process, illustrated in FIGS. 2–4, via holes, such as via hole 202, are etched through an ILD 204 to interconnection lines or substrate 206 formed below. Next, thin adhesion layer 308, such as TiN, is generally formed over ILD 204 and into via hole 202, as shown in FIG. 3. Next, a conformal tungsten film 310 is blanket deposited over adhesion layer 308 and into via 202. The deposition is continued until via hole 202 is completely filled with tungsten. Next, the metal films formed on the top surface of ILD 204 are removed by chemical mechanical polishing, thereby forming metal vias or plugs 110 shown in FIGS. 1 and 4.
FIG. 4 is a side view schematic cross-section illustrating a via after chemical mechanical polish removal of the excess tungsten. All excess tungsten 310 and adhesion layer 308 have been removed and the via 110 is flush with ILD layer 204. Via 110 fills the gap between layers in ILD 204. Via 110 may contact the top of an interconnection line or a substrate 206.
In a typical chemical mechanical polishing process, a substrate or wafer is placed face-down on a polishing pad which is fixedly attached to a rotatable table. In this way, the thin film to be polished is placed in direct contact with the polishing pad. A carrier or chuck is used to apply a downward pressure against the backside of the substrate or wafer. During the polishing process, polishing pad, and the table on which the polishing pad is mounted, are rotated. The substrate is also rotated by a motor coupled to carrier. An abrasive and chemically reactive solution, commonly referred to as a “slurry”, is deposited onto the polishing pad during polishing. The slurry initiates the polishing process by chemically reacting with the film being polished. The polishing process is facilitated by the rotational movement of the polishing pad relative to the wafer, and rotation of the wafer on the polishing pad, as slurry is provided to the wafer/pad interface. Polishing is continued in this manner until all of the film on the wafer is removed.
Slurry composition is an important factor in providing a manufacturable chemical mechanical polishing process. Several different tungsten slurries have been described in literature. One slurry available is Commercial Tungsten Slurry: Semi Sperse W2000 available from Cabot Corporation/Microelectronics Materials Division of Aurora Ill. It has been found that slurries support a chemical reaction of the material being polished in addition to assisting with the physical removal of the material from a substrate by physical means. Many slurries contain an abrasive such as silica SiO2 or alumina Al2O3 to remove oxidized material from a substrate.
When tungsten is placed in water there is a spontaneous reaction generating an oxidation product. The tungsten and water react to form tungsten oxide, hydrogen ions and free electrons. The reaction may be described as:W+3H2O⇄WO3+6H++6e−E0=0.19 v  ((1))The oxidation potential, E0, of equation (1), which is 0.19 v, indicates that this is a spontaneous reaction. However, while this reaction occurs spontaneously, it is not very fast. The generation of the hydrogen ions indicates, however, that this is an acidic reaction. To enhance the reaction rate of the oxidation of the tungsten, hydrogen peroxide is added to the CMP chemical environment. The hydrogen peroxide acts as an oxidizing agent providing a capability to accept electrons. This reaction may be described by the equation:H2O2+2H++2e−⇄2H2O E0=1.77 v  ((2))The balanced reduction oxidation (“redox”) equation is shown below:W+3H2O2+3H2O+6H+⇄WO3+6H++6H2O E0=1.96 v  ((3))Removing common terms, which will still somewhat accurately describe the reaction, reduces the equation to:W+3H2O2⇄WO3+3H2O E0=1.96 v  ((4))The reaction described in equation (4) has a much higher reaction rate than the reaction described in equation (1).
The oxidation product of equation (4), WO3, is unfortunately not soluble to a sufficient degree to allow removal of the oxidation product so fresh metal may be oxidized. To accelerate removal of the reaction product, an abrasive is added to the slurry. The abrasive may be silica or alumina particles. The abrasive particles physically remove the oxidation product from the metal layer clearing the way for additional oxidation of the metal to be removed.